1. Field of the Invention
The present invention relates generally to spray cooling thermal management systems and more specifically it relates to a coldplate spray cooling system that provides high heat evaporative cooling of electronic component hotspots.
2. Description of the Related Art
Liquid cooling is well known in the art of cooling electronics. As air cooling heat sinks continue to be pushed to new performance levels, so has their cost, complexity, and weight. Liquid cooing is replacing air cooling and enabling the performance of electronics to grow exponentially.
A significant performance enhancing feature of liquid cooling relates to the possibility of cooling localized high heat flux zones, commonly referred to as hotspots. Electronic components create varying amounts of heat across their surfaces and a varying amount of heat as a function of time. Today's microprocessors, for example, may be constructed on a silicon die roughly 1 cm by 1 cm. As shown by FIG. 2, The die may have multiple zones for different functions. Such zones may be for inputs and outputs (I/Os), level 1 cache, level 2 cache, and the core. The core may be roughly 0.5 cm by 0.5 cm and is where the main computer processing takes place. Although the core may be, but is not limited to, roughly only 25% of the total area of the die, the core creates almost the entire heat generation of the chip and may create a hotspot. Wherein a chip might be rated for an average heat load of 110 watts, with an average heat flux of 110 watts per centimeter squared, the core may generate 100 watts of that heat and have a heat flux of 400 watts per centimeter squared.
There are a number of different liquid cooling system styles. A first style is bare die. This method places a dielectric liquid coolant in direct contact with the die of a component to be cooled. Heat is directly transferred from the component to the fluid. Another style is referred to as coldplate cooling. This method uses a dielectric or non-dielectric cooling fluid contained within a housing. The housing is in direct contact with a component to be cooled and the fluid is indirectly thermally connected. Typically, a thermal interface material is sandwiched between the housing and the component package, but may also be between the housing and the component (no lid). Although bare die cooing is typically more efficient than coldplate cooling, primarily due to reduced thermal resistances between the fluid and the heat generating transistors, coldplate cooling provides a more flexible system capable of being used with standard chip packages. Coldplate cooling also provides the ability to use a wide range of cooling fluids. Both styles of liquid cooling styles may be used with single-phase and two-phase cooling systems.
Single-phase liquid cooling systems, such as U.S. Pat. No. 6,234,240, have a thermal management block containing a pure liquid. The thermal energy of the electronic component is transferred to the cooling liquid by means of sensible heat gains. The heat transfer rate of the system is equal to the heat transfer coefficient of the cooling fluid on the surface to be cooled, multiplied by the contact area, and further multiplied by difference in temperature between the contact surface and the cooling fluid. For low cost cooling system using ambient temperature cooling fluids, it can be easily seen that a heat transfer improvement requires a faster mass flow rate of the coolant, an increase in the heat transfer coefficient, or an increase in contact area. Even by the common single-phase practice of increasing the contact area by adding fins, pins and the like, single-phase thermal management blocks are unlikely to absorb anything above low-to-medium heat fluxes. Although low-to-medium heat fluxes across a multiple square inch contact area may equate to the overall heat generation rate of an electronic component, low-to-medium heat flux single-phase systems are not capable of localized performance required to cool component hotspots.
The preferred method of liquid cooling is two-phase cooling. With these systems, energy is absorbed by the cooling fluid as latent heat gains. Due to the increased energy required for a phase change in comparison to sensible heat gains, two-phase cooling systems offer the ability to provide more compact and higher performance cooling systems than single-phase systems.
An exemplary two-phase cooling method is spray cooling. Spray cooling uses at least one pump for supplying fluid to at least one nozzle that transform the fluid into droplets. These droplets impinge the surface of the component to be cooled and can create a thin coolant film. Energy is transferred from the surface of the component to the thin film. Because the fluid may be dispensed at or near its saturation point, the absorbed heat causes the thin film to turn to vapor. This vapor is then condensed, often by means of a heat exchanger, or condenser, and returned to the pump. A doctoral dissertation researched and authored by Tilton entitled “Spray Cooling” (available through the University of Kentucky library system, 1989), describes the physics behind spray cooling and the creation of thin evaporative films capable of absorbing heat in excess of 800 watts per centimeter squared. U.S. Pat. No. 5,220,804 discloses a wide area spray cooling system utilizing a vapor management protrusion.
Recently, the problem of cooling hotspots has led to new two-phase cooling technologies. One such technology is disclosed by U.S. Pat. No. 6,443,323, describing a method of variably cooling a computer component through the use of incremental sprayers. The incremental sprayers deposit fluid onto each zone at a mass flow rate necessary for complete phase change. Drops are ejected from an orifice in serial. Although this method improves the efficiency of the system, that is in attaining complete phase change of all dispensed fluid, the dispensing method does not provide spray characteristics necessary to create high heat flux thin film evaporative cooling and high performance cooling of hotspots.
Another method of cooling hotspots is two-phase microchannels, such as described by U.S. Pat. No. 4,450,472. Although this method does not use spray cooling, the design does provide the ability to remove heat in the range of 400–1000 watts per square centimeter using water. The system discloses a method of placing a very small microchannel array over a component. Although this method can effectively lower the temperature of the core, due to large pressure drops and resulting size limitations the method does not efficiently address the needs of the other areas of the die. In addition, this method does not efficiently cool multiple hot spots in separate parts of the die, such as multi-core processors.
For the foregoing reasons, there is a need for a liquid cooling solution that effectively cools the one or more hotspots of a computing component. Thus, there is a need for a localized cooling solution capable of large heat fluxes. Also, the high heat flux cooling system must efficiently and reliably cool the other non-high heat flux areas of the chip. The resulting cooling solution would allow significant improvements in processor performance.